1. Field of the Invention
This invention relates to a video display system for the presentation of graphics and more particularly to a system for expanding the graphic capabilities of a terminal in an independent manner.
2. Discussion of the Context of the Invention
Computer-based information systems have now evolved to the stage where it is both desirable and feasible to allow the public access to the wealth of information stored in private or public data bases using commonly available display devices and communicating via existing channels. "Viewdata" or "videotex" are generic terms which have been used to describe systems enabling two-way, interactive communication between the user and the data base, generally communicating via telephone lines and using an ordinary but specially adapted television set for display of pictorial information. "Teletext" is another generic term used to describe one-way communication from the data base to the user, with transmission being accomplished in portions of the broadcast T.V. spectrum, and display again being on a specially adapted T.V. Both system types must have a large range of flexibility, since a number of alternatives exist with respect to various system components. For example, although a television set may be a preferred display device for home use, different terminals may access the data base in an office environment, such as bi-level (plasma) displays, and other special purpose CRTs. Additionally, other communication channels, such as dedicated coaxial, twisted pair cable, or satellite or land-based radio may interconnect the users who input or output information from the data base, and each type of channel has its own requirements and limitations.
In view of the fact that different types of equipment and facilities must interact to achieve satisfactory overall results, several attempts have been made to standardize the manner in which information (primarily pictorial as opposed to text) is encoded and decoded. The success of these systems, which include one described by the Communications Research Center of the Canadian Department of Communications, in CRC Technical Note No. 699-E published November 1979 in Ottawa, Canada, must be measured against several parameters. First, the procedure used to encode the pictorial information must make reasonably efficient use of the bandwidth available in the communication channel and the processing capability of the microprocessor usually located in the user's terminal. Second, the users of the system must, during both encoding and display operations, have a high degree of control and flexibility in specifying how the information will be processed. Finally, the techniques used must recognize that different equipment--particularly displays--will be used, some having non-standard resolution and other capabilities, and that all must operate satisfactorily using the same encoding/decoding strategy.